1. Field of the Invention
The present invention concerns a magnetic resonance system of the type having a transport element, an examination tunnel or bore and an outer (external) antenna arrangement.
2. Description of the Prior Art
Magnetic resonance systems of the above type are known wherein by means of the transport element, an examination subject can be inserted into the examination tunnel and removed from the examination tunnel, the examination tunnel being surrounded by the outer antenna arrangement and wherein the examination subject inserted into the examination tunnel can be excited to magnetic resonance by means of the outer antenna arrangement, and magnetic resonance signals excited in the examination subject can be received by means of the antenna arrangement. Such magnetic resonance systems also are known that have auxiliary arrangement that can be disposed inside the examination tunnel, and that also form a tunnel that, with auxiliary arrangement inside the examination tunnel, is parallel to the examination tunnel. With the auxiliary arrangement inside the examination tunnel, the transport element still can be inserted into the tunnel of the auxiliary arrangement the examination tunnel and removed therefrom. The auxiliary arrangement has an internal antenna arrangement that surrounds the tunnel of the auxiliary arrangement and by means of which magnetic resonance signals excited in the examination subject can be received.
Such magnetic resonance systems are described, for example, In U.S. Pat. No. 5,311,134. When magnetic resonance signals excited in the examination subject are received with the outer antenna arrangement, the received magnetic resonance signals exhibit a relatively low signal-to-noise ratio. The signal-to-noise ratio normally is smaller the larger the cross-section of the examination tunnel.
To improve the reception quality, the use of local coils (for example head or knee coils) are also is known. Local coils provide a better signal-to-noise ratio, but only from a small reception area.
Local coils are complicated in their handling, relatively prone to interference, and also affect the examination subject (for example, a person to be examined). Furthermore, they can be used only when the magnetic resonance signal of interest arises exclusively from the examination subject that is adjacent to the local coil.
By contrast, when a larger part of the subject should be examined (for example for overview images), the receipt of the magnetic resonance signal ensues conventionally by means of the outer antenna arrangement, in spite of the relatively low signal-to-noise ratio.
From U.S. Pat. No. 5,311,134 and also from European Application 0 580 327, it is known to use a small inner (internal) antenna arrangement inserted into the examination tunnel instead of the outer antenna arrangement.